Electric charge

Key Points

When we see a spark or a bolt of lightning, we are seeing the result of negative charges moving through the air. These negative charges are called electrons.

Like charges repel. Unlike charges attract.

Only negative electrons move. The positive particles, called protons, are fixed in the centre of atoms.

The unit of electrical charge is the coulomb (C).

Protons and electrons carry charge

Atoms are made up of neutrons, protons and electrons. Neutrons have no charge. Protons are positively charged, and electrons are negatively charged. In a neutral atom there are exactly the same number of protons and electrons, so the charges add up to zero.

In metals (conductors), electrons can move freely, while the protons and neutrons are fixed in place within the nucleus of the atoms.

Some other non-metallic substances, such as carbon and water, also conduct electricity. Charges may also exist in liquids and gases. A battery has fluid electrolytes, which will carry charged atoms (ions) to electrodes, providing a chemical means of storing electrical energy. Gases may also consist of charged ions.

Insulators

A polythene rod can be charged by rubbing briefly on material. Notice than electrostatic forces are much stronger than gravity.

Plastic, glass, natural fibres and wood, are insulators. Insulators do not have free electrons, but hold their electrons in place. When they are rubbed against another material, some electrons may pass from one material to another.

Rubbing a polythene rod with wool will cause the polythene to become negatively charged. Other materials, such as perspex, become positive when rubbed.

Charging insulators by friction

Charged rods. Like charges attract, unlike charges repel

Two charges of the same sign (like charge) will repel (push each other away). Two charges of different signs (unlike charge) will attract.

If a perspex rod and a polyethene rod are brought close to each other, they will attract, because one is positive and one is negative.

The charge is induced because there is no contact with the sphere. Instead, the close proximity of the charged rod causes the electrons on that side of the sphere to be repelled, and so are pushed to the other side. The result is that one side of the sphere has a net negative charge, and the other a net positive charge.

Similarly, when a positively charged insulator is brought close to a metallic sphere, there is an induced negative charge on the sphere near to it, and the far side becomes positive, since the electrons are attracted to the positive rod.

Discharge

If a positively charged rod touches a metallic sphere, it takes away some of the electrons, leaving the sphere with a net positive charge.

If there is enough negative charge built up during inductance, when the positively charged side is close enough, some of the electrons can jump across the gap. This is an electrical discharge.

Lightning is an example of an electrical discharge. And when we become charged on a dry day walking on a synthetic carpet with rubber or plastic soled shoes, we may feel a discharge when we touch something metallic, like a door handle.

When electricity flows, it is a movement of electrons. Each electron has a precise and unchanging charge. The amount of electrical charge is therefore the number of electrons that will be conducted.

Hair stands on end when we touch a Van de Graaf electrostatic generator because the positive charge of the ball will pass along the outside of our bodies and into the hair strands. Because they have like charge, they repel each other.

History of Electrostatic Machines

The history of electricity is a fascinating story, which followed landmark discoveries. There is a series of machines which led to the modern world which is totally dependent on electrical power to function.

Hauksbee Generator

Hauksbee generator, invented around 1703-5 was an early charge generator

Invented by Francis Hauksbee in 1703-5, this was one of the earliest instruments used to generate electric charge on call in a laboratory. It utilises friction between an insulator (glass jar) and a band which was rubbed against it by means of a rotating wheel, to generate static charge, which could be drawn off via copper wires.

Leiden Jar

Also known as the Leyden jar, and developed by the Dutch scientist Pieter van Musschenbroek in 1745, this instrument used the Hauksbee generator to store charge in a jar of water. It led to the accidental discovery of electrical circuit when Moeschenbroek touched the electrode while holding the jar in the other hand. It is named after the town in Holland where the discovery was made.

Leyden Jars in parallel to create the first battery

The modern equivalent of the Leiden jar is the ubiquitous capacitor, which is an electronic component used to control and regulate the movement of charge around a circuit.

Benjamin Franklin, in 1752, increased the charge by drawing lightning into a wine bottle, thus proving that lightning was the same sort of electricity as the static electricity produced by the laboratory Leiden jar.

Franklin is also attributed with inventing the term 'battery', which he used to describe a set of Leiden jars wired together.

Leyden jar with a dischage wand to close the circuit safely

Van de Graaff Machine

This machine is a development of the first device to generate electrical charge: the Hauksbee Jar. This device uses a band to rub an insulator and generate charge through friction. This charge is accumulated around a large metal sphere. Touching this sphere while standing on an insulator allows the human body to take on the charge, causing effects like hair standing on end.

A smaller sphere brought near to a projection from the Van de Graaff sphere demonstrates lightning. The voltage generated is a surprising 20,000 volts! But the current that flows is as small as 0.000 02 amps, which is far too little to cause any injury.